The development of the polymerase chain reaction (PCR) enabled the use of DNA amplification for a variety of uses, including molecular diagnostic testing. There are challenges associated with the use of PCR for molecular differential diagnostic (MDD) assays, however. PCR utilizes specific primers or primer sets, temperature conditions, and enzymes. PCR reactions may easily be contaminated, primer binding may require different conditions for different primers, primers should be specific for a target sequence in order to amplify only that target sequence, etc. This has made it even more difficult to amplify multiple sequences from a single sample.
Diagnostic testing of clinical samples to find one or more causative disease agents has, in the past, required that microorganisms be isolated and cultured. This may take days, however, and in many cases a diagnosis must be acted upon within hours if the patient's life is to be saved. Analysis of a single clinical sample to identify multiple organisms in order to determine which one(s) may be the causative agent(s) of disease is the desired method for MDD, and methods have been developed to better achieve that goal. For example, multiplex PCR methods have been developed to amplify multiple nucleic acids within a sample in order to produce enough DNA/RNA to enable detection and identification of multiple organisms. Multiplex PCR has disadvantages, however. For example, each target in a multiplex PCR reaction requires its own optimal reaction conditions, so increasing the number of targets requires that the reaction conditions for each individual target are less than optimal. Furthermore, multiple sets of high-concentration primers in a system often generate primer dimmers or give non-specific, background amplification. This lack of specificity also requires the additional steps of post-PCR clean-up and multiple post-hybridization washes. Crowded primers reduce the amplification efficiency by requiring the available enzymes and consuming substrates. Differences in amplification efficiency may lead to significant discrepancies in amplicon yields. For example, some loci may amplify very efficiently, while others amplify very inefficiently or fail to amplify at all. This potential for uneven amplification also makes it difficult to impossible to accurately perform end-point quantitative analysis.
One method utilizes nested gene-specific primers used at very low concentrations to enrich the targets during the initial PCR cycling. Later, common primers are used to amplify all the targets. The entire reaction is performed in one tube, no additional rounds of PCR are required, and it does not require specialized instruments but may instead be performed using regular thermal cyclers. There are disadvantages to this method, however. For example, because a low concentration of primers is used to enrich the targets during the initial cycles, the sensitivity of the assay is ultimately decreased, the initial enrichment cycles require longer annealing time for each cycle, and the enzyme is more likely to be less efficient over the number of cycles required to amplify the target.
A need still exists for more sensitive, faster, and more efficient methods for amplifying DNA and/or RNA from multiple targets to promote rapid identification of those targets.